This invention relates to a monitoring system for sensing the d-c bus current translated from the output of an a-c to d-c converter and through a low pass filter to a voltage source inverter.
D-C bus current monitoring is employed in an inverter system for a variety of different purposes. For example, the sensed bus current may be compared with a set point current, representing a desired bus current level, to provide a control or error signal which effects automatic regulation of the bus current to maintain it at the desired level. Measuring the bus current is also useful to detect fault conditions, such as a shoot-through fault in the inverter or a short circuit in the load driven by the inverter. When a fault occurs, causing the sensed bus current to abruptly increase above a preset maximum allowable level, both the converter and inverter may be shut down to avoid damage to the inverter or to the load. As still another example, bus current monitoring is useful in power factor control.
Sensing of the d-c bus current is usually achieved by means of a d-c current transformer in the d-c bus. In some prior systems, a current shunt is added to the d-c bus. In any event, these prior bus current monitors are relatively complex and expensive, particularly when it is desirable to electrically isolate the sensing circuitry from the power circuitry. In sharp contrast, the d-c bus current monitoring system of the present invention is isolated from the power circuitry and yet it is of extremely simple construction and is very inexpensive compared to the previous monitors.